The present invention relates to an ignition system for an internal combustion engine.
A conventional ignition system as is disclosed in U.S. Pat. No. 3,424,944 comprises a first thyristor connected in series with the primary winding of an ignition coil, a second thyristor having the cathode thereof connected to the cathode of the first thyristor and a capacitor coupling the anodes of the first and second thyristors. A DC power source supplies a DC current at a constant voltage. The first thyristor is turned on in advance of the ignition timing of the engine. The current through the primary winding is established. Meantime, the capacitor is charged through a path including the first thyristor. At the ignition timing, the second thyristor is turned on and the capacitor is discharged through the second thyristor to apply a reverse voltage across the anode and cathode of the first thyristor, so that the first thyristor is turned off. The turn-off of the first thyristor leads to sudden decrease of the primary current, which in turn causes generation of a high voltage in the secondary winding.
It is first noted that this conventional ignition system is intended to be used in an engine provided with a DC power source of constant voltage such as batteries and does not operate in an engine which is provided with an AC generator instead of batteries.
Secondly, as the output voltage from the DC power source is constant the voltage applied to the primary winding is constant regardless of the engine speed. Inductance and resistance of the primary winding and of any element in series circuit with the primary winding as well as the voltage applied to the primary winding determine the length of time required for the primary current to be established, or in other words, to reach a value sufficient to cause, when suddenly reduced to nil, a high voltage in the secondary winding. Since the inductance, the resistance and the voltage applied are all constant in this conventional system, the time required for establishing the primary current is constant regardless of the engine speed. On the other hand, the time required for one cycle of operation becomes shorter as the engine speed increases. As a result, at engine speeds which are too high for the primary current to be fully established, a voltage generated in the secondary winding is insufficient. This means that the engine speed may be limited by the factors of the resistance and the inductance as well as the voltage.
Another factor that may limit the engine speed is the time required for charging the capacitor. The capacitor has to be charged to a voltage which is sufficient to turn off the first thyristor when the capacitor is subsequently discharged through the second thyristor to apply a reverse voltage across the anode and cathode of the first thyristor. In this conventional system, the capacitor is charged by an oscillator and this charging begins when the first thyristor conducts. Output voltage of the oscillator is constant and therefore the time required for completing the charging is constant regardless of the engine speed. As the engine speed becomes so high that time allotted for the full charging of the capacitor is insufficient, the first thyristor is not turned off by the reverse voltage from the capacitor which is not sufficient, with the result that the primary current is not interrupted, and ignition of the engine does not take place. Consequently, this may again limit the engine speed.